fix(client): better error reporting on login/registration

[xvello]

Closes #2030 - unless you downgraded api.atuin.sh by mistake, this is misleading error reporting on non-200 responses:

• Use the handle_resp_error helper on the registration and login code paths for consistency. Tune its strings to be a bit more generic, feel free to reword
• Only check the server version if the request is a 200 (if we get a 503 from the CDN, the version check would run and wrongly report)
• Remove the version fallback from ensure_version if the header is not preset. Since we bumped the major version, the fallback is not needed + the header has been present for 6 months

Tangentially related, should handle_resp_error directly call ensure_version now, to ensure a consistent behaviour?

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[xvello]

Yes, my weekend has been very rainy, thank you very much ☔

[ellie]

lgtm, thank you!

Tangentially related, should handle_resp_error directly call ensure_version now, to ensure a consistent behaviour?

I imagine this would make more sense